In the field of lithographic printing plates, socalled presensitized (PS) plates comprising an aluminum support having provided thereon a light-sensitive layer composed of a light-sensitive composition have hitherto been employed. The aluminum support to be used generally has a surface roughened by various processes hereinafter described. If necessary, the roughened surface is subjected to etching with an acid or alkali aqueous solution, followed by anodic oxidation, and further followed, if desired, by surface treatment for rendering the aluminum surface hydrophilic, to thereby produce a support for a lithographic printing plate. A light-sensitive layer is provided on the support to form a presensitized lithographic plate, which is then subjected to exposure to light, development, retouching, gumming, etc., to produce a printing plate. The resulting printing plate is mounted on a printing machine to carry out printing.
The processes for surface roughening as above referred to include mechanical processes, such as ball graining, wire graining, brush graining, liquid honing, etc., an electro-chemical process called electrolytic graining, a chemical process, and a combination of two or more of these techniques. However, each of these techniques has respective disadvantages, such as set forth below.
In the case of ball graining, high skill is required for selection of the kind of material and the size of balls, control of a water content in carrying out abrasion, determination of the abrasion time, and evaluation of the finished surface, particularly as characteristic of the nature of a batch system. Besides, productivity of the ball graining process is very low.
In the case of wire graining, the roughness of the resulting surface of the aluminum plate is non-uniform. In the case of brush graining, high roughness cannot be obtained, and the grained surface tends to show scatter in roughness due to wear of the abrasive brush used. Further, the strong friction between the brush and the abrasive and the aluminum plate causes complicated scratches on the aluminum surface to form many sharp projections like molding flash. These projections cause problems, such that the light-sensitive layer of the presensitized printing plate that should be removed by development is likely to remain to some extent, causing stains on the plate face, and that scratches are easily formed on the aluminum surface through rubbing or sliding upon handling of the supports.
In the case of liquid honing, in which a slurry liquid having dispersed therein a fine abrasive powder is sprayed onto the aluminum plate at a high rate by compressed air, etc., the fine abrasive powder easily sticks to the aluminum surface to thereby form projections. Further, the surface roughness cannot be increased to a fully satisfactory extent because of the low impulsive force of the slurry liquid against the aluminum surface. Another disadvantage of this technique is that the jet nozzle wears significantly because the slurry liquid is jetted at a high rate.
When the electro-chemical process is employed for surface roughening, it is necessary to strictly control electrolysis conditions so as to maintain the surface roughness attained constant. Further, this technique not only involves a rather large consumption of electric energy, but also entails great cost for disposal of the waste liquor containing Al ions accumulated in the electrolyte. Finally, the chemical roughening process is not suitable for mass production because the time required for treatment is relatively long, and also great expenses are incurred in disposal of the waste liquor similarly to the case of the electro-chemical process.
In an attempt to overcome some of the above-described disadvantages associated with the graining process, there have been proposed an improved process comprising a combination of the brush graining or wire graining and the electrolytic graining as disclosed in U.S. Pat. No. 2,344,510 and Japanese Patent Application (OPI) Nos. 123204/78, 145701/78, and 63902/79 (the term "OPI" as used herein means "unexamined published application"), and an improved process comprising a combination of the liquid honing and the electrolytic graining as disclosed in Japanese Patent Application (OPI) No. 19593/85. According to such combined processes, however, when the brush graining is adopted as a first step, i.e., a mechanical graining step, stains are apt to be formed in the non-image areas during printing, and when the wire graining is adopted as the first step, the printing plate has poor printing durability. In the case of using liquid honing as a first step, the resulting printing plate, though exhibiting resistance to stains, is still not fully satisfactory in printing durability, thus leaving room for further improvements in order to satisfy the market requirements for more excellent qualities.